Artemis is older and not so swish as Arcadia but it is easy to get to anywhere on the ship and you are more likely to meet people more than once.
We were unsure when we first went on board but we found it grew on us. Being smaller it may be be able to get closer into places in Norway than Arcadia would.
Hope this helps.
I found the standard cabins on Artemis to be a bit cramped and the bed layout was far from ideal as one was folded away during the day, so it depends on what you go for. The brochure or P&O web site may provide plans and photos to illustrate this. As far as children go, there aren't as many on Baltic trips as there are on those to the med so I doubt any ship going north would be over run with them.
Pretty obviously the Arcadia is a much larger, more modern ship. It has a lot more space and a lot more venues to visit on board. It has a proper tiered theatre instead of just a show lounge. The choice of eating places is more extensive. There are nearly twice as many people onboard also. I am sure that the amount of poolside/sunbathing space must be greater on the Arcadia if you are interested in that? . The Artemis is a more tradional cruise ship, where as the Arcadia is more flashy and modern. The choice is yours? Check out the prices for the time you want to go. Maybe this could make up your mind? If you haven't cruised before then you would probably be very happy on the Artemis, but you would have more space (especially in the cabins) on Arcadia. Study the P&O brochure and make up your minds. Happy cruising.
Many thanks or your reply. Very helpful, kindest regards.
MANY thanks for the time taken by all who have replied; very interesting points to consider - extremely helpful.